To remove PMs in exhaust gas emitted through an exhaust passage of an internal combustion engine to the outside of a vehicle, an exhaust gas purification system has been used in which a DPF that traps PMs is provided in the exhaust passage of the internal combustion engine. With the increase in amount of PMs trapped in the DPF, the pressure drop across the DPF increases to deteriorate the fuel-efficiency, or the amount of PMs which exceed the limitation of the PM trap and which slip through the DPF increases to deteriorate the emission performance. For these reasons, a forced regeneration treatment for the re-combustion of the trapped PMs is performed on the DPF by raising the temperature of the DPF at regular intervals to reduce the amount of PMs emitted to the atmosphere. By employing the DPF, the amount of PMs emitted to the atmosphere is reduced to a level of about 1/100.
However, as shown in FIG. 11, the DPF has a problem in that a phenomenon occurs in which the amount of PMs emitted to the atmosphere temporarily increases (the DPF outlet soot concentration increases in FIG. 11) because of “PM slip-through (soot break-through)” in which PMs pass through the DPF after the re-combustion of PMs in the forced regeneration treatment on the DPF. This phenomenon is thought to occur as follows.
For example, in a wall flow type DPF, when PMs pass through a wall surface that sections cells, the PMs are trapped on the wall surface. Here, PMs with relatively large particle sizes do not pass through the wall surface, but are attached to a front side (upstream side) of the wall surface to form a surface filtration cake layer. The formation of the surface filtration cake layer increases the pressure drop to some degree, but simultaneously the PM trap efficiency is also improved. However, the surface filtration cake layer is combusted and lost by the PM re-combustion in the forced regeneration treatment on the DPF, and hence the PM slip-through (blow off) occurs in which the PM trap efficiency is temporarily lowered. As a result, PMs are not trapped by the DPF, but slip through the DPF, so that the amount of PMs emitted to the atmosphere increases temporarily.
Note that, when the temperature of the DPF raised by the forced regeneration treatment drops after the PM re-combustion, PMs are again trapped by the DPF to form the surface filtration cake layer again on the front side of the wall surface. With the growth of the surface filtration cake layer, the PM trap efficiency increases, which leads to the decrease in the PM slip-through amount, and in turn, the decrease in the amount of PMs emitted to the atmosphere.
The degree of the deterioration in the PM removal due to the temporary increase in the amount of PMs emitted to the atmosphere caused by the PM slip-through is not detectable with eyes. However, the PM slip-through may cause the deterioration in the K-factor, which is a deterioration factor listed in an application for the certification of the emission performance, and the like. Hence, a countermeasure has to be taken against the PM slip-through.
As a countermeasure against the PM slip-through, the present inventors have proposed a DPF regenerator as described in Japanese patent application Kokai publication No. 2011-149357, for example. Specifically, to reduce the soot break-through in which PMs slip through a DPF because of the decrease (blow-off) in the PM trap efficiency immediately after DPF regeneration in an exhaust gas apparatus in which the DPF is provided in an exhaust gas flow passage of an internal combustion engine, post injection which is performed in a forced regeneration of the DPF is prohibited and the forced regeneration of the DPF is stopped at a time point where the PM concentration detected downstream of the DPF is not lower than a predetermined value and the blow-off occurs in the DPF. Thus, the temporary increase in the amount of PMs emitted to the atmosphere due to the post injection is suppressed.
In this DPF regenerator, the post injection of the forced regeneration treatment on the DPF is stopped at the same time as the occurrence of the blow-off in the DPF. This prevents PMs generated because of the post injection performed for the forced regeneration treatment on the DPF from being emitted to the atmosphere during the blow-off immediately after the PM re-combustion. Thus, the increase in the amount of PMs emitted to the atmosphere is suppressed.